In recent years, to improve the efficiency of transport of crude oil and natural gas, increasing the inside pressure of pipelines has been studied. Along with this, a higher strength is being demanded from steel pipe for line pipe. Furthermore, high strength steel pipe for line pipe is also being required to offer toughness, deformability, arrestability, etc. For this reason, steel plate and steel pipe made of mainly bainite and martensite formed with fine ferrite have been proposed.
For example, see Japanese Patent Publication (A) No. 2003-293078, Japanese Patent Publication (A) No. 2003-306749, and Japanese Patent Publication (A) No. 2005-14640.7. However, these relate to high strength steel pipes of the American Petroleum Institute (API) standard X100 (tensile strength 760 MPa or more) or better.
On the other hand, improved performance is being demanded from the high strength steel pipe of the API standard X70 (tensile strength 570 MPa or more) or API standard X80 (tensile strength 625 MPa or more) currently being used as material for trunk pipelines. As opposed to this, the method of heat treating the heat affected zone (HAZ) of steel pipe having a base metal comprised of bainite in which fine ferrite is formed so as to improve the deformability and low temperature toughness has been proposed. For example, see Japanese Patent Publication (A) No. 2004-131799.
In this way, the method has been proposed of starting from steel plate and steel pipe, mainly comprised of bainite and martensite and achieving both strength and toughness, and promoting the formation of ferrite so as to improve the deformability and other properties. However, recently, there has been increasingly stronger demand for low temperature toughness. Toughness of the base metal at the ultralow temperature of −60° C. or less is being sought. Further, the low temperature toughness of not only the base metal, but also the HAZ is extremely important.